First generation highly cross-linked ultra-high molecular weight polyethylenes (UHMWPE) are generally irradiated and melted to reduce the adhesive/abrasive wear of UHMWPE components in total joint arthroplasty (see Muratoglu et al., J Arthroplasty, 2001. 16(2): p. 149-160; Muratoglu et al. Biomaterials, 1999. 20(16): p. 1463-1470; and McKellop et al., J Orthop Res, 1999. 17(2): p. 157-167), The post-irradiation melting step, used to impart oxidation resistance to irradiated UHMWPE, generally reduces the fatigue strength of irradiated polyethylene by about 20% (see Oral et al, Biomaterials, 2004. 25: p. 5515-5522).
It is generally known that mixing of polyethylene powder with an antioxidant prior to consolidation may improve the oxidation resistance of the polyethylene material. Antioxidants, such as vitamin E and β-carotene, have been mixed with UHMWPE powder or particles by several investigators (see, Mori et al p. 1017, Hand-out at the 47th Annual Meeting, Orthopaedic Res Soc, Feb. 25-28, 2001, San Francisco, Calif.; McKellop et al. WO 01/80778; Schaffner et al EP 0 995 450; Hahn D. U.S. Pat. No. 5,827,904; Lidgren et al. U.S. Pat. No. 6,448,315), in attempts to improve wear resistance. Mori el al. also described that irradiation does not decrease the oxidation resistance of antioxidant-doped polyethylene. The investigators (see, McKellop et al. WO 01/80778; Schaffer et al, EP 0 995 450; Hahn D. U.S. Pat. No. 5,827,904; Lidgren et al. U.S. Pat. No. 6,448,315) described mixing polyethylene powder with antioxidants, followed by consolidating the antioxidant-powder mix to obtain oxidation resistant polyethylene. Mixing of the resin powder, flakes, or particles with vitamin E and consolidation thereafter result in changes in color of polymeric material to yellow (see for example, U.S. Pat. No. 6,448,315). In addition, the addition of the antioxidant to the UHMWPE prior to irradiation can inhibit crosslinking of the UHMWPE during irradiation (Parth et al., J Mater Sci-Mater Med, 2002. 13(10): p. 917-921: Oral et al., Biomaterials, 2005. 26: p, 6657-6663). However, crosslinking is needed to increase the wear resistance of the polymer.
Vitamin E-stabilized highly cross-linked UHMWPE is a next generation highly cross-linked UHMWPE and has been developed (see Oral et al., Biomaterials, 2004. 25: p. 5515-5522; Muratoglu et al., Transactions of the Orthopaedic Research Society, 2005. 1661; Oral et al., Transactions of the Orthopaedic Research Society, 2005. 1171, Oral et al., J Arthroplasty, 2005. in print) to decrease the extent of mechanical and fatigue strength degradation seen in first generation irradiated and melted highly cross-linked UHMWPEs. Melting in combination with irradiation creates cross-links and facilitates recombination of the residual free radicals trapped mostly in the crystalline regions, which otherwise would cause oxidative embrittlement upon reactions with oxygen. However, cross-linking and the decrease in the crystallinity accompanying post-irradiation melting are thought to be the reasons for the decrease in fatigue strength, yield strength, ultimate tensile strength, toughness and elongation at break of radiation cross-linked and melted UHMWPE. It is, therefore, desirable to reduce the irradiation-created residual free radical concentration in cross-linked UHMWPE without reducing crystallinity, so as to achieve high fatigue resistance for high stress application that require low wear.
An antioxidant can be used to interact with the free radicals induced by irradiation and prevent them from reacting with other chains to further the oxidation cascade. This eliminates the need for post-irradiation melting of radiation cross-linked UHMWPE and avoids the decrease in crystallinity and strength accompanying post-irradiation melting. Vitamin-E (α-tocopherol) is such an antioxidant and protects irradiated UHMWPE against oxidation. However, for a long-term oxidative stability of an irradiated implant, vitamin E must be present throughout the component at all times.
Previously, high temperature doping with subsequent high temperature homogenization at ambient pressure was used to enhance α-tocopherol diffusion in irradiated UHMWPE (see Muratoglu et al., U.S. application Ser. No. 10/757,551, filed Jan. 15, 2004; and Oral et al, Transactions of the Orthopaedic Research Society, 2005, 1673). This method is suitable for doping of finished components. However, the duration of doping and homogenization increases considerably with increasing component thickness. Therefore, it would be desirable to accelerate the rate of α-tocopherol diffusion in irradiated UHMWPE, which was not possible with prior art practices. This invention would also allow the incorporation of antioxidants into bar stock efficiently, from which medical implants can be machined.